Radio warning system



Oct. 31, 1944. H. w. APPEL Y RADIO WARNING SYSTEM Filed Feb. 27. 1942Patented @ein 3i, lifascites anni@ wma sys'rawr Henry W. lippel, KewGarden Hills, N. Y., assignor to Radio Corporation of America, acorporation of Delaware Application February 27, 1942, Serial No.432,549

13 Claims.

My present invention relates to radio warning systems, and moreparticularly to an improved and highly efficient form of alert receivercapable of emitting an intermittent siren signal upon reception of apredetermined sub-audible control frequency.

There has been disclosed by H. B. Deal, in application Serial No.403,736, filed July 23, 1941, ,February 1, 1944, as U. S. Patent No.2,340,798, a radio Warning system wherein sub-audible modulation wassuperimposed upon normal modulation at the transmitter, and responsiverelays were employed at the receiver. These relays were so arranged thatone sub-audible modulation tone controlled the receiver reproducercircuit, while another sub-audible modulation tone rendered reproductionineffective. In that patent selective reeds were utilized.

It has also been proposed by G. L. Beers, in application Serial No.432,607 filed February 27, 1942, to use only a single sub-audiblecontrol tone which is transmitted during an alert period.`

A warning signal is provided by permitting the audio amplifier tooscillate whenever the control tone is being transmitted. An electronicdevice is employed to silence the audio amplifier in the absence of thesub-audible modulation tone, While another electronic device is employedto render the first electronic device inoperative upon the reception ofthe sub-audible modulation tone.

It may be stated that it is one ofthe important objects of thisinvention to provide a radio warning system of the type using modulationtones of different sub-audible frequencies for rendering operative orinoperative an audio oscillation network employing a device forproducing an intermittent siren effect. l

Another important object of my invention is to improve the audiofeedback circuit of the aforementioned Beers system and to provide afeedback path between the voice coil of the reproducer and the inputcircuit of the first audio amplifier, the feedback path being designedto produce an audible signal in the low frequency portion of the-audiorange. t

Another object of my invention is to provide in Still other objects ofthis invention are to improve generally the simplicity and efficiency ofradio .warning systems and more especially to provide such systems in acompact, economical and readily-assembled manner.

The novel features which I believe to be char acteristic of my inventionare set forth with particularity in the appended claims; the inventionitself, however, as to both its organization and method of operationwill best be understood by reference to the following description takenin connection with the drawing in which I have indicateddiagrammatically a circuit organization whereby my invention may becarried into effect.

Referring now to the accompanying drawing, there is shown the circuitdetailsof a radio receiving system which includes the improved net-Works and devices. In general, the receiving system comprises receivercircuits designed to receive modulated carrier waves such as aretransmitted from standard broadcast stations. For example, the receivingsystem can be the usual type of superheterodyne receiver employed in thebroadcast band of 550 to 1700 kilocycles (kc). However, it is to beclearly understood that the present invention is not limited to thatband and the receiver, by proper choice of tuned circuits, may operatein the short wave bands. Furthermore, the invention is not restricted toamplitude modulated carrier waves, since it may be used in connectionwith frequency modulated, or phase modulated, carrier waves.

Considering rst the networks of the receiver, the numeral I denotes aconverter stage which "may be a combined first detector-local oscillatorcollector device may be of any desired type. The

an alert type of receiver a self-generated siren which comprises anaudio feedback circuit'employing a device which is capable of varyingthe tone of the siren at a predetermined low freintermediate frequency(I. F.) energy produced in the output circuit of the converter istransmitted to an I. F. amplifier. The latter is designated by thenumeral 2 and is provided with a resonant output circuit 3' tuned to theoperating I. F. value. The following resonant circuit 4 is magneticallycoupled to circuit 3, and both circuits may be tuned to the aforesaid I.F. value. The latter may be of the order of 400 kc., or any otherdesired value. Reference is made to the aforesaid Beers application fora disclosure of circuts which may be employed in networks l and Thenumeral designates a tube of the diodetriode type. The common cathodecooperates with the auxiliary anode 6 to provide a demodulator. orsecond detector, device. The anode 6 is connected to the high potentialside of. input circuit 4, the low potential side of the latter isconnected to ground through a path which includes a resistor element 1and a potentiometer resistor 8. The path 1-8 functions as the diodeload. Direct current voltage developed thereacross is utilized forautomatic volume control (AVC) and the AVC bias is applied over lead 9to the signal grids of the converter and I. F. ampliiier tubes.Pulsatlon voltage filter resistors I6 and l0' are inserted in the AVClead. The action of the AVC circuit is well known.

The resistor element 1 is shunted by a pair of condensers and l i' whilethe cathode of tube 5 is grounded and connected to the junction ofcondensers il and Il'. The audio voltage developed across the diode loadresistor is applied to the audio signal grid |2 of tube 5. 'I'.ie gridl2 is connected to the adjustable contact element i3 of potentiometerresistor 3.. This connection path includes condensers |4 and |5 andresistor I6. The junction of condensers I4 and |5 is Iconnected toground by resistor. l1. This last mentioned network, between adjustableelement I3 and grid I2, effectively prevents the transmission of V,anycarrier component to the audiolgrld.

The grid return path for the audio section of tube 5 may be tracedthrough the path comprising resistor I8, lead i9, the secondary winding26 of audio output transformer 2|, lead 22, resistor 23 and resistor 24to ground. At this point it may be stated that the voice coil 25 of theloudspeaker reproducer 26 is connected in shunt across secondary winding26. 22 is by-passed to ground by condenser 22. It will, therefore, beobserved that I have provided an audio feedback path from the voice coilof the reproducer to the grid circuit of the audio ampliiler fed by thedemodulator.

Returning to the normal path of signal transfer through the audioamplifier network, the amplified audiovoltage is developed across plateload resistor 21. The numeral 2B denotes the audio power output tube.For the sake of compactness tube 28 is shown as one ofthe type includingwithin a common envelope the electrodes `of a beam power tube, and theelectrodes of a diode rectier. Specifically tube 28 is of the '10 L'IGTtype. The signalinput grid 29 is corinected to ground by grid returnresistor 35. The cathode 3| is connected tofground through two paths.One of these paths may be traced from lead 32 to the contact point 33 ofa control switch, the flexible contact element 34 thereof, lead 35 andbiasing resistor 36 to ground. The by-pass condenser 31 by-passesresistor 36 for audio frequency currents. Hence, resistor 36 providesnormal bias for grid 29. The second path from cathode 3| to groundconsists of lead 32, lead 38, switch 39, lead 40, lead and biasingresistor 36. Hence, it will be seen that when switch 39 is open, thenthe cathode circuit of the power amplifier tube has been opened. This isthe alert position of the receiving system. Closure of switch 33, 34,due to reception of the proper sub-audible modulation control tone, willthen close the cathode circuit of the second audio am plifler circuit.When switch 39'is in its closed position, then normal reception ofprogram modulation on the received carrier waves is had.

The lead ,sary to describe the construction of the beam amplier sectionof tube 28, since those skilled in the art are fully aware of suchconstruction` The plate 43 thereof is connected to the upper end of theprimary winding of output transior'mer 2i. The cathode 44 of the diodesection of tube 28 is connected to an intermediate point 45 on theprimary winding, While the anode 46 is connected through a resistor tothe ungrounded side of the power input line 41. As schematicallyrepresented, the filaments of the various tubes are shown includedbetween ground and 41 so that the laments are heated from the powerline. W

The grounded side of the power line includes a volume control switch topermit control over the energlzation of the entire system. The primarywinding of transformer 2| is shunted by the by-pass condenser 48. Theintermediate tap 45 is connected by lead 49 to the voltage supplypotentiometer which comprises the resistor sections 50, 5| and 52arranged in series between ground and lead 49. Appropriate by-passcondensers to ground are employed at the potentiometer sections. .Itwill, therefore, be seen that the rectiler 46, 44 acts to rectify thealternating current from the power line, and that the rectified voltage,after proper iiltration, is employed to supply the various potentialsfor the system. Thus, the plate circuits of the tubes in stages l and 2,tube 5 and plate 43 are all supplied with positive potential from thelead 53.

There will now be described the control relays which function todetermine whether the siren tone shall be reproduced or not. Twoelectromagnetic relays are employed. One of them is the o" relay, whilethe other is the "on relay. The iirst named relay comprises a yoke 54which is provided with a magnetizing winding 55'. One end of winding 55is connected bylead 56 to a metallic contact 51. The opposite end ofwinding 55 is connected'by lead 58 and a resistor 53 to the energizinglead 45. The contact 51 is a 'part of a switch whose mobile element is areed 59 constructed to vibrate solely at 24 cycles. Asshown in thedrawing, the upper end of the reed terminates in a contact point whichcompletes the electric circuit through ground upon contact betweenelements 51 and 59. The lower end of the reed is xedly secured to the,support Gil which is horizontal, and the latter is carried by a verticalspring element 6| whose lower'end is rigidly secured to chassis of thesystem. An L'shaped armature 62 has the right end of its horizontalsection rigidly secured to the exible member 6|. The armature 62 isadapted to be attracted toward the electromagnet 63, which may beconstructed in the manner of theelements 54, .55. The energizing windingof electromagnet 63 has one end thereof connected to the junction ofpotentiometer sections 5|, 52. The opposite end of the winding isconnected by lead 64 to the plate 65 of one of the triodes oftwin-triode tube 66. The tube 466 is included in a noise gate networkwhich will be described aaeaeee frequency of 36 cycles, and is denotedby the numeral 61. As in the case of reed 59, the reed 61 has its upperend functioning as a switch element for the contact stud 68. 'I'he stud69 is connected by lead 69 to one end of the energizing winding 1li ofthe magnetic yoke 1l. The opposite end of winding is connected throughresistor 59 to the energizing lead 69. It will now be seen that closureof switch 61, 69 energizes winding 19 which causes magnetization of yokel l. v

The yoke 56 has pivotally associated therewith a metallic armature 12which is normally biased away from the yoke by spring 18. It is believedthat further description of the relay elements is unnecessary, since itis of well known construction and it is suiiicient for the purposes ofthis application schematically to represent the relay. The yoke 1I issimilarly provided with a pivoted armature lll which is spring-biasedby' spring 15 to position away from the yoke. The free ends of armature12 and M are so relatively constructed that when the free end ofarmature 18 is pulled into contact with yoke 1l-,. the free end ofarmature 12 will ride over the free end of armature 19 due to thebiasing action of spring 13. of a device to guard against accidentalrelease of armature 14 during such periods when the yoke 1l is supposedto be energized. Oficourse, when yoke 59 is energized to pull armature'l2 towards it. the armature 1 6 will readily be capable of release.

The armature 14 is constructed so as to control three switches. One ofthese switches is 33, 34. and the latter is the middle one of a bank ofswitches. The lowermost switch in the bank comprises the fixed contact8l] which cooperates with the flexible contactor 8l. The uppermostswitch comprises the xed contact 82 which cooperates with the flexiblecontactor 83. The ends of each of contactors 83. 34 and 8l are fixed onone side. while the opposite ends are xedly secured to spaced points ofan insulation member 84. The lower end of insulation member 86 isrigidly secured adjacent to the free end of armature 14. Conseouentlyarmature 19 and the spaced contactors 83, 34 and 3| move as a singleunit. This is a well known form of relay switching construction and willbe well understood by those skilled in the art.

Considering now the electrical circuits to each of the switches,contacter 83 is connected byI a lead 85 to the ungrounded side of thepower line, while Contact point 82 is grounded. An outlet plug 86,adapted for any other control service.

may be included in the line 85. 1t will, therefore, be seen that theswitch 83, 82 is provided for the purpose of an auxiliaryindicator'service which may be energized through the outlet plug 86. Ithas already been explained that switch 36. 33 functions to control theeffectiveness of the cathode circuit of the amplifier section of tube28. The contactor 8l of the lowerx'nost switch is connected by lead 61to the feedback lead i9. The contact point 89 is connected through aswitch 88 to ground. The switches 88 and 39 are represented as arrangedfor concurrent closing or opening. It will therefore be seen thatlswitch 80, 8|, upon closure, completes a path through lead 81 and switch88 to ground, if switch 88 is closed. This path will act toshort-circuit the audio feedback path Hence, armature 12 functions inthe manner are selectively operated by magnet 63.

should the listener throw switches 98 and 89 into the closed receivepositions.

As has been stated heretofore, the audio oscillations produced due tothe audio feedback are intermittently interrupted. The intermittentoperation is provided by a neon oscillator circuit which includes theneon tube 9i) one of whose electrodes is connected to ground through theresistor 9|, while the opposite electrode is connected by lead 92 to thepositive voltage line 53. The grounded electrode of neon tube 98 isfurther connected through condenser 99 and lead 95 to the junction ofresistors 29 and 26. Rel sistor 9i is shunted by condenser 93. Thenetwork 9|, 93 provides a time constant circuit for the neon oscillator.Since the laments of the various receiver tubes are arranged in serieswith the-rectifier 66, 99, failure of any of these filaments will beindicated by neon tube 99. Hence, the neon tube functions to performseveral operations. In the first place, it acts to produce interruptionof the siren tone; secondly, it acts to visually indicate whether thereceiver tubes are energized, and lastly, it indicates intermittentoperation by virtue of its flashing on and off at the period ofinterruption of audio feedback.

As explained previously, the. reeds 59 and 61 In order to energizemagnet 63 it is merely necessary to derive the modulation controlvoltage from the second vdetector circuit, and use such voltage toenergize the windingof relay 63, 62. However, there is provided incircuit with the demodulator load4 resistor a noise gate which guardsrelay 63. 62 from operation by noise impulses. This noise gate circuitis not a part of my present invention, and, therefore, it is believedthat a general description of this circuit will be sufficient for thepurposes of this application.

The rectified sub-audible tones are taken off bv lead 96 from the anodeend ofk resistor 1. Filters. of the appropriate design, reduce theundesired program'audio components, and apply to the grid 91 of thefirst triode section the sub- 'audible tone with any superposed noisepulses. The grid 91 is coupled to lead 96 by condenser 98. the gridreturn resistor 99 connecting the grid 91 to ground. 'Ihe cathodes ofboth triode sections of tube 66 are connected to ground. The plate |99of the rst triode section is connected to the positive voltage supplysource through a voltage reducing resistor lill. The output of the firsttriode is applied to the grid 65' of the second triode through couplingcondenser |92. The plate 65 of the second triode is connected throughlead 6d to the 'energizing winding of elcctromagnet 63.

The noise pulses present at the grid 91 are derived from the demodulatordiode with a negative polarity, and on aonlicaton to grid 91A produceplate current cut-off in that tube. The effective .positive voltage onplate |00 is made small to make the plate current cut-off take place ata low value of grid voltage, and also to limit the `plate current 'ofthis tube. A double limiting action is 'thus produced. The second triode`of tube 66 funct'ons as an amplifier of the limited voltage. The outputcurrent flowing from plate 65 through lead 66 energizes the d'ivirvgcoil of electromagnet 63 and this sets the armature 62 in motion. Thisin turn vibrates the reeds. Of course, only that reed will vibrate whichis tuned to the frequency of the current flowing through the drivingcoil. The diode output voltage may vary linearlywith percent modulation,but the limiter output is substantially constant above 25 modulation.The reed actuating voltage developed at plate 65 follows the samevariation as the output voltage at plate indicating a constant outputabove 25% modulation. -A reed voltage of 12 volts (peak to peak) is justsufficient to operate the relay system. This is much less than themaximum output applied to the reeds. 'I'his wide tolerance is desirableto take care of 'possible manufacturing variations in tuned reedfrequency.

'I'he reeds may vibrate slightly under noise interference impulses. Toprevent contact in the case of each of the reed contacts, thedistance'between each pair of contacts must be made large enough. Adistance of @fof an inch was chosen as a satisfactory compromise betweenthe reed -motion required for contact and the spacing necessary to avoidnoise actuation. This eliminates the possibility of false signaling. A24 cycle reed is chosen to operate the off" relay, while a 36 cycle reedis selected for the on relay. The reeds are very sharply tuned toresonate at their specified frequencies, and have a selectivity factorQ" of about 250. The use of the armature type drive keeps the reeds outof the magnetic circuit. This prevents any-influence of the magneticfield of 63 on the tuned frequency of each reed.

, It will be understood that each of members 60, 6| and 62 are metallicwhereby the closure of the reed contacts provide energizing circuits foreach of relay windings 55 and 10. A constant direct current in the reedwinding of relay 63 is necessary for proper operation of the vibratingarmature 62. This is supplied by the plate current of the output sectionof tube 66. The broadcast sub-audible tone, taken off from the detector,causes that reed to vibrate which is tuned to the tone frequency. Thereed amplitude builds up to a large resonant swing, and the reedcontacts are closed, or sparked, at every swing of the energized reed. Acapacitor H0 is connected in shunt with relay winding 55, and acapacitor is connected in shunt with winding 1li. each capacitorbeing-charged up from the power supply capacitor-every time itsrespective reed contact is made. Each relay capacitor discharges throughits associated. relay winding at a peak current intensity ofapproximately 38 mlliamperes. 'I'his current is sufficient for positiverelay action. f

Considering now the operation of the system, it will be understood thatthe manual switches 39, 88 are actuated to open position to place thereceiver in the alert state. The power supply switch has been closed,and 'hence the filaments are all heated which means that the receiverhas been conditioned for the control frequencies. At the. broadcasttransmitter there will be applied either the 24 cycle or 36 cyclemodulation tone to the carrier. Assuming that the 36` cycle modulationtone has been applied to the carrier, the collected modulated carrierwave is received. converted to the I. F., amplied at the I. F., and thendetected. Sub-audible signal sensitivity of the receiver is consideredto be the minimum antenna input necessary to operate a relay with 5%modulation at 36 cycles. The 5% level represents a conservative ratingfor sensitivity. It is expected that broadcast stations will modulate inthe region of twenty or thirty percent. The detected 36 cycle componentis fed over lead 96 to the noise gate network.' Any noise impulses willbe eliminated, as explained previously. The 36 cycle frequency currentflowing with constant amplitude in the circuit connected to plate 65will energize the driving winding of relay 63. As a result armature 62will be vibrated with the consequent actuation of reed 61 suiiicient toclose the electrical circuit through relay winding 1,6.

Energization of winding 10 results in attraction of armature 14 againstthe bias of spring 15. This results in closing of each of switches 83,82, 34, 33 and 8|, 80. concurrently, the free end of armature 12 willride over the free end of armature 14 thereby acting as a keeper forthelatter. Accordingly, the Operator at the transmitter need only apply the36 cycle modulation to the carrier for a short period of time, say 5seconds. This follows from the fact that at the receiver attraction ofarmature 14 will cause the free end of armature 12 positively to lockarmature 14 in place.

Closure of switch 83, 82 permits plug to be used. Any alarm device notrequiring more than three peak amperes may be plugged into 86. 'I'hisprovision will be useful for installations requiring a visual warningsignal, or an audible signal at locations remote from the receiver.Closure of switch 34, 33, as explained previously, closes the cathodecircuit of the power amplifier tube whereupon audio feedback from thevoice coil to the grid circuit of the first audio amplifier is permittedto provide the audio oscillations which are reproduced by reproducer 26.A distinctive siren tone of the order of some 300 cycles results. Byproper choiceof the coupling elements in the audio circuit, such ascondenser 4I and the inherent low frequency resonance `of voice coil 25produces a typical siren wail The audible tone thus generated is variedin frequency by means of the neon tube oscillator which is permanentlyin circuit. The siren tone shifts up and down at a period ofapproximately 0.5 cycle per second. This intermittent low pitched sireneffect is distinctive and highly desirable for devices of thischaracter. l

It is necessary to have a switching provision to out off the alarmsignal in order to hear voice modulation from the speaker. Hence, themanually operable switch 39, 88 is provided so that the operator canthrow the switch into receive position. As explained previously, closureof switch 88 short-circuits the audio feedback path, and effectivelyremoves the siren.` Hence, the

.reproducer will be able to reproduce the voice modulation of thebroadcast station, with any instructions that it is desired to transmit.Hence, it will be seen that switch 39. 88 provides a means for receivingvoice modulation without having to radiate from the transmitter the 24cycle off tone. When the 24 cycle tone is radiated, and afterldemodulation at the receiver, reed 58 will be vibrated and ycauseenergization of winding 55. Armature 12 will be pulled from its keeperposition. Accordingly, the armature 14 will be pulled away from yoke 1Idue tospring 15. Of course, it is only necessary to apply the 24 cyclevmodulation to the carrier for a short period of assise vention is by nomeans limited to the particular organization shown and described, butthat many modifications may be made without departing from the scope ofmy invention, as set forth in the appended claims.

What I claim is:

l. A method of signaling which includes modulating a carrier wave with acontrol frequency, transmitting the modulated wave, deriving the controlfrequency from the transmitted wave at a reception point, producingoscillations of audible frequency at said point in response to receptionof said control modulated carrier wave, and interrupting: the productionof oscillations at a relatively low period.

2. A method of signaling which includes modulating a, carrier wave witha sub-audible control frequency, transmitting the modulated wave,deriving the control frequency from the transmitted wave, producingoscillations whose frequency is in the low portion of the audiol rangein response to said control frequency, generating auxiliary oscillationsof a relatively low frequency, and utilizing the auxiliary oscillationsto interrupt said first oscillations.

3. A method of radio warning signaling with a radio receiver of the typeincluding a demodulator and an audio amplifier followed by aireproducer,which includes transmitting to the receiver a modulated carrier wavehaving included in its modulation a sub-audible modulation tone,demodulating the modulated carrier wave, transmitting the demodulatedWave energy through the audio amplifier, producing oscillations ofaudible frequency in response to the said modulation tone, providinginterruption oscillations of a relatively low period, and utilizing thelatter oscillations to interrupt the production of said audibleoscillations.

4. A method of signaling air raid warnings which includes receiving acarrier wave modulated with audio frequency components, said waveincluding a control frequency of a predetermined 5. In a radio warningsystem, a modulated carrier wave transmission network, a demodulator, afirst audio amplifier, a second audio amplifier, a reproducer having avoice coil coupled to the output of the second audio amplifier, an audiofeedback circuit connected between said voice coil and the inputelectrodes of said first audio amplifier to provide oscillations .of anaudible frequency, a neon tube oscillator circuit operatively associatedwith said feedback circuit intermittently to interrupt saidoscillations, and means responsive to the existence of a control signalof a predetermined frequency in the modulation on the received Wave forcontrolling said feedback.

6. A method of signalling which includes modulating a carrier wave witha first control frequency, transmitting the modulated wave. derlv ingthe control frequency from the transmitted wave at a reception point,producing oscillations of audible frequency at the said point inresponse to the derived control frequency, interrupting the productionof oscillations at a relatively low period, and replacing the firstlcontrol frequency by a different control frequency to render saidoscillation production ineffective.

7. A metnod of signalling which includes modulating acarrier wave with anist sub-audible control frequency, transmitting the modulated wave,deriving the control frequency from the transmitted wave, producingoscillations whose frequency is in the low portion of the audio range inresponse to said derived control frequency, generating auxiliaryoscillations of a relatively low frequency, utilizing the auxiliaryoscillations to interrupt said first oscillations, and preventing saidoscillation-production in response to a frequency change of said controlmodulation.

8. A method of radio Warning signaling with a radio receiver or' thetype including a demoduiator, an audio amplifier and a reproducer; comprising transmitting to the receiver a modulated carrier wave havingincluded in its modulation a Iii-st sub-audible modulation tone,demodulating the modulated cariier wave, transmitting the demodulatedwave energy through the audio ainpliiier, producing oscillations ofaudible frequency in response to tne said modulation tone, providinginterruption oscillations of a relatively low period, utilizing thelatter oscillations to interrupt said audible oscillations thereby toprovide an undulating audible signal, and replacing said firstsub-audible tone with a second sub-audible tone of different frequencyat the transmitting step thereby automatically to stop said audiblesignal.

9. A method of signaling an air raid warning which includes modulating acarrier wave with a sub-audible control frequency, transmitting themodulated wave, deriving the control frequency from the transmitted waveat a receiver, producing oscillations of audible frequency and of theorder of 300 cycles in response to the derived control frequency, andinterrupting the production of oscillations at a relatively low periodthereby to provide a siren effect.

10. A method of signaling which includes modulating a cariier wave witha sub-audible control frequency, transmitting the modulated wave `to a.receiver, deriving the control frequency from 4 the transmitted wave,producing oscillations at the receiver whose frequency is in the lowportion of the audio range 4in response to said control frequency,interrupting the oscillations at a relatively low frequency, andreplacing said subaudible control modulation by a different controlmodulation of different sub-audible frequency to stop said oscillations.

11. A method of radio warning with a radio receiver of the typeincluding a demodulator and an audio amplin'er followed by a reproducer,which includes transmitting to the receiver a modulated carrier wavehaving included in-its modulation a sub-audible modulation tone,demodulating the modulated carrier wave, transmitting the demodulatedwave energy through the audio amplier, producing oscillations of audiblefrequency in response to the said modulation tone, providinginterruption oscillations of t a relatively low period, utilizing thelatter oscillations to interrupt the derivation of said audibleoscillations, and transmitting to the receiver a to replacement of saidmodulation frequency by a diiferent modulation frequency.

13. A receiver provided with atleast a demodulator, audio channel andsound reproducer; the improvement comprising normally ineffective meansfor providing regenerative feedback in said audio channel, a lowfrequency oscillator circuit for intermittently preventing saidfeedback, and means, responsive to a modulation control tone indemodulator output, for rendering said feedback effective.

I-ENRY W. APPEL.

